For example, a fixed type constant velocity universal joint can be taken as an example of a constant velocity universal joint used as means for transmitting a rotational force from an engine to wheels of an automobile at a constant velocity. The fixed type constant velocity universal joint has a structure in which two shafts, a driving shaft and a driven shaft, are coupled to each other and which is capable of transmitting rotational torque at a constant velocity even when the two shafts form an operating angle. Generally, a Birfield type (BJ) constant velocity universal joint and an undercut-free type (UJ) constant velocity universal joint have been widely known as the above-mentioned constant velocity universal joint.
As illustrated in FIG. 48, the fixed type constant velocity universal joint of the BJ type includes: an outer race 3 having an inner spherical surface 1 in which a plurality of track grooves 2 are equiangularly formed along an axial direction, the outer race 3 serving as an outer joint member; an inner race 6 having an outer spherical surface 4 in which a plurality of track grooves 5 are equiangularly formed in pairs with the track grooves 2 of the outer race 3 along the axial direction, the inner race 6 serving as an inner joint member; a plurality of balls 7 interposed between the track grooves 2 of the outer race 3 and the track grooves 5 of the inner race 6, for transmitting torque; and a cage 8 interposed between the inner spherical surface 1 of the outer race 3 and the outer spherical surface 4 of the inner race 6, for retaining the balls 7. In the cage 8, a plurality of pockets 9 for housing the balls 7 are arranged along a circumferential direction.
At an inner surface of a hole portion of the inner race 6, a female spline 10 is formed. A male spline 12 of a shaft 11 is fitted into the hole portion of the inner race 6, to thereby fit the male spline 12 to the female spline 10 of the inner race 6. Then, a stop ring 13 for preventing dropping is fitted to an end portion of the male spline 12.
In order to close an opening portion of the outer race 3, an end cap 15 and a flexible boot 16 are mounted. The flexible boot 16 is formed of a bellow 18 made of a rubber material or a resin material, and a cylindrical adapter 19 made of a metal. The bellow 18 includes a small diameter end portion 18a fitted onto the shaft 11, a large diameter end portion 18b connected to the adapter 19, and a bent portion 18c provided between the small diameter end portion 18a and the large diameter end portion 18b. The small diameter end portion 18a of the bellow 18 is fixed to the shaft 11 by being fastened by a band 20. The adapter 19 includes a swage portion 19a swaged and fixed to the large diameter end portion 18b of the bellow 18, and an annular flange portion 19b fitted to the outer race 3. Note that, the end cap 15 includes a main body portion 15a as a bottomed short cylindrical member, and an annular flange portion 15b fitted to the outer race 3.
Further, a curvature center O2 of each of the track grooves 5 of the inner race 6 is shifted from a joint center O on the end cap side in the axial direction. Further, a curvature center O1 of each of the track grooves 2 of the outer race 3 is shifted from the joint center O on the flexible boot side in the axial direction. That is, the curvature center O2 of the track groove 5 of the inner race 6 and the curvature center O1 of the track groove 2 of the outer race 3 are offset to opposite sides in the axial direction by equal distances F and F with respect to the joint center O.
Conventionally, there has been proposed an 8-ball constant velocity universal joint (Patent Literature 1) in which the ball diameter is reduced for the purpose of achieving high performance and reducing the size, so as to suppress reduction in load capacity per one ball. Further, there are proposals of forming the tracks of the inner and outer races into a spiral form or tilting the tracks in the axial direction, to thereby set the adjacent tracks to be plane-symmetrical. Thus, the contact force between the ball and the track portion of the inner or outer race can be reduced, thereby enhancing durability (Patent Literature 2).